In recent years, the Global Positioning System (GPS) has provided a significant advancement in satellite communications. Aircraft of various types are major users of the GPS system, and high speed aircraft have unique antenna requirements. The equipment that is required on the aircraft in order to efficiently utilize the GPS signals includes an antenna that must provide a right hand circular polarization and a uniform pattern coverage over virtually all of the upper hemisphere. By providing a uniform amplitude response over a wide coverage region, the receiver is able to maintain a signal lock to the GPS satellites with a useful signal to noise ratio.
High speed aircraft that maneuver extensively often abruptly change their look angles to the GPS satellite. Thus, a wide beam width coverage is necessary for the receiver to be able to track as many satellites as possible while still maintaining a proper Geometric Dilution of Precision (GTOP). Avoiding aerodynamic drag is an essential feature of most high speed aircraft, and it is equally important in many cases to provide an antenna that does not require significant structural modification of the aircraft. Slot antennas have been developed and used in GPS applications, largely in recognition of the characteristics that GPS antennas must exhibit when installed in high speed aircraft. Slot antennas are particularly desirable where low profile or flush installations are needed, as they are in high speed aircraft. A variety of slotted antennas have been proposed, including cylindrical slot antennas that are provided with helical slots. The prior antennas have included four slots and have generally been described as a quadrifilar slot antennas that have used micro strip feed systems. While this type of antenna has been found to be generally satisfactory in many applications, it is less than ideal in some respects, particularly in its horizon coverage and antimulti-path capabilities.